Boilers are required in many applications, worldwide, for the production of hot water and saturated and superheated steam. Hot water has numerous applications, including space heating with central boilers. Saturated steam is primarily used for process steam, heating, and air conditioning, whereas superheated steam is primarily used in steam turbine applications for the generation of electrical power. A typical boiler consists of three heat absorbing sections: a radiant section, a convective section, and possibly a superheater section. The radiant section is the heat transfer surface area exposed to the flame in the combustion chamber (sometimes called the “furnace” or the “firebox”), whereas the convective section is the heat transfer surface area shielded from the flame, but excluding any superheater. The radiant section and the convection section each comprise riser tubes connecting the water drum to the steam drum. The superheater section is located in either the high temperature section of the boiler convection section or at the rear of the boiler. Saturated steam generated in the radiant and convective sections of the boiler is collected in the steam drum and then, at the saturation pressure and temperature, is transferred to the superheater section of the boiler, where it is raised to a higher temperature while remaining at approximately constant pressure.
The oxidization of a fuel in the combustion chamber of a boiler generates thermal energy that is transferred to the water in the various sections of the boiler and thereby creates the steam that is generated inside the radiant and convective sections. The efficient conversion of the energy in the fuel into steam is of practical importance, as is the cost of the boiler system.
A fundamental advantage of package boilers is an installed cost which is considerably lower than that of a field-erected boiler. This cost advantage is made possible by basic designs that allow standardized processes for fabricating multiple units in a factory, while providing sufficient flexibility to permit straightforward, efficient, and technically sound adaptation to the specific needs of a particular application.
The configuration of the tubes connecting the lower drum to the upper drum is especially important in a package boiler. These tubes must not only convey saturated steam and water to the upper drum, but must also liberally cool the furnace and the walls in order for the boiler to have its small size. This is critical in a package boiler because the space available for insulation is limited.
Package boilers are described in various publications and patents. These include Steam/Its Generation and Use (40th edition), published in 1992 by The Babcock and Wilcox Company, at pages 25-8 and 25-9; Combustion Fossil Power (4th edition), published in 1991 by Combustion Engineering, Inc. (especially pages 8-36 to 8-42); Keystone Steam Generating Systems, a brochure published in 1978 by Zurn Industries, Inc. (especially pages 1 to 7); Type A Boiler, a brochure published by The Bigelow Company of Madison, Conn.; and Dang U.S. Pat. No. 5, 881,551 (especially column 1, lines 35 to 65).
These references show various basic configurations for the tubes connecting the lower drum (or drums) and the upper drum. Steam/Its Generation and Use, Combustion Fossil Power, and the Dang patent disclose D-type, two-drum boilers, in which the upper drum is located directly above the lower drum adjacent one side wall of the boiler. (As used herein, “two-drum” means having two drums and only two drums.) D-type boilers are not symmetrical and thus are inherently unbalanced, due to the heavier weight of the drums on one side relative to the weight of the tubes on the other side. Consequently, transporting a D-type package boiler requires the addition of counterweights, or shifting the boiler laterally on the transporting vehicle, to bring the center of gravity of the load to the center of the truck, railcar, barge, or other vehicle. Combustion Fossil Power also discloses an A-type, three-drum package boiler, in which the upper drum is located above and midway between two lower drums adjacent opposite side walls of the boiler, stating “Its symmetry makes the A design ideal for rail shipment because ballast is not needed. This simplifies off-loading and handling”. The Bigelow brochure discloses another A-type boiler. The Zurn brochure discloses an O-type package boiler, in which the upper drum is located directly above the lower drum midway between the side walls of the boiler. While O-type boilers are symmetrical and thus inherently balanced, the arrangement of the drums tends to limit the height of the combustion chamber. Also, in A-type and O-type boilers two symmetric arrays of tubes enter opposite sides of the upper drum, which precludes optimum location of the entrances of the tubes, particularly the radiant tubes, radially about the drum. Moreover, it will be understood that the respective basic tube configurations also have numerous other characteristics which positively or negatively affect boiler design and/or performance.
The prior art also discloses offset, two-drum, field-erected boilers in which the lower drum is on one side of the boiler and the upper drum is on the other side. These include Wojan U.S. Pat. No. 477,880 and Marshall U.S. Pat. No. 2,648,316. In these boilers the tubes connecting the drums pass between the combustion chamber and the upper drum to permit the tubes to enter the drum at entrances spaced uniformly about the lower half of the drum, which limits the ability of their tube configurations to take advantage of the offset two-drum concept. The same is true of the package boilers described in the English Boiler, Inc. brochure “A New Generation of Boilers/‘S’-Type Boiler”, prepared Nov. 30, 2001.